1. Field of the Invention
The present invention generally relates to lenses and more particularly to a focusing lens of a new type.
2. Prior Art
Graded index rod lenses can be used to focus the output of a laser diode to an elliptical spot. A graded index rod lens also can be used to efficiently couple a laser diode to an optical fiber if the divergence angle that the fiber can accept is as great as the maximum divergence angle of the output beam of the diode. Spherical ball lenses can achieve the same functional result as graded index rod lenses. Multiple cylindrical lenses, arranged in combination, can perform similarly. In some cases, however, lenses are omitted and optical fibers are simply placed near the emitting surface of the diode in a way that allows a substantial portion of the light to be captured by the fiber core.
Graded index lenses cannot make the light beam from a laser diode more circular without discarding a large fraction of the light, and they cannot remove the astigmatism from an astigmatic beam. If the lens images essentially all of the light from the diode, the maximum divergence angle of its output beam will be approximately the same as that of the diode and it will therefore be no more compatible with an optical fiber of low numerical aperture than the unfocused output of the diode is. Moreover, hermetic glass-to-metal sealing to a graded index lens, as is required in some sealed systems, will likely destroy its focusing property, because the high temperature used in fusing the solder glass will activate diffusion between zones of differing refractive index within the lens material and from the sealing glass into the glass of the lens and change the refractive index gradient of the lens.
Spherical ball lens systems require that multiple components be assembled to make up the focusing system and the results are no better than with the graded index rod lens. They cannot easily be sealed into the diode package. Cylindrical lenses also require assembly of multiple components and are not easily built into the diode package or hermetically sealed.
Accordingly, there is a need for an improved type of lens which can focus light from an astigmatic laser diode in an improved manner.
Preferably, the lens should be produced as a single piece of material requiring no assembly to perform its function. It should focus the light from an astigmatic laser diode to a very small spot and make highly elliptical beams less elliptical. The maximum divergence angle for the output beam should be smaller than that of the input beam for highly elliptical beams, resulting in more efficient coupling to fibers of small numerical aperture. The lens should be compact and simple. The optical system required to fully correct and focus the output of a laser diode should be capable of being made very compact, simple and efficient with the use of the lens. The lens should be capable of being hermetically sealed to a diode package using solder glass. The index of refraction of the material near the lateral surface should have little or no effect on the performance of the lens.